Separation of dialkyl benzenes



Patented Aug. 22, 1950 UNITED STATES PATENT OFFICE SEPARATION OF DIALKYL BENZENES Leland K. Beach, Mountainside, and Charles E. Morrell, Westfleld, N. J., assignors to Standard Oil Development Company, a corporation of Delaware No Drawing.

Application lglay 2, 1946,

Serial No. 666,83

3 Claims.

.tures is an expensive and tedious operation if A further object is to provide an efllcient method of separating the isomers by selective sulfonation in cooperation with other steps, such as fractional crystallization and fractional hydrolysis.

The selective sulfonation of one dialkyl benzene component in preference to the sulfonation of an isomeric component mixed therewith can be demonstrated to be influenced by certain factors, as follows:

(1) Increase in proportion of isomer mixture to the sulfonating a ent favors preferential sulfonation of one of the isomers.

(2) Increased dilution of the sulfonating acid by sulfonation product favors preferential sulfonation.

Also, increased concentration of the isomer less susceptible to sulfonation favors separation of unsulfonated isomers from sulfonated isomers. In any event one isomer is not exclusively sulfonated or separated from the other isomer in a practical operation.

The factors of excess hydrocarbon feed to sulfonating agent, dilution of the sufonating acid by sulfonation product, and dilution of the hydrocarbon reactant for favoring selective reaction are contrary to general practices in ordinary sulfonation procedures. Moreover, it will be appreciated that selective sulfonation as practiced for the purposes of the present invention may be considered as partaking of the nature of a solvent extraction treatment wherein the unreacted isomers constitute the raflinate and the absorbed o1" reacted isomers form the extract which is miscible with the sulfonating acid.

For the purposes of this invention, proper steps are taken to obtain a high yield of rafiinate with increased content of the higher freezing point isomer relative to the initial isomer mixture.

The raillnate removed from the acid extract of 'sulfonated isomers upon being cooled to moderately low temperatures precipitates pure crystals of the higher freezing point isomer. It can be seen that by having the raffinate contain a.

larger amount of the higher freezing point isomer, the temperature of crystallization is made higher, thus reducing operation difficulties and refrigeration requirements.

The sulfonated isomer extract in acid solution upon being subjected to a controlled steam distillation yields separate isomer fractions of fairly high purity. The fraction rich in higher freezing point isomer is also suitable for the fractional crystallization alone or together with the rafiinate for separation of a pure isomer product.

The mother liquors obtained in the fractional crystallization down to moderately low temperatures are again sub ected advantageously to selective sulfonation for a repeated removal of one of the isomers.

Some of the advantages of the method described are: recovery of individually concentrated or pure isomers under more practical conditions; elimination of waste; and economy in refrigeration.

It is to be understood that the present invention is useful in the isolation and purification-of various dialkyl benzene isomers for which mand p-xvlenes, diethyl benzenes, and other Ca to C10 dialkyl benzenes serve as examples.

For illustration of how the principles set forth are used, selective sulfonation of mixed mand p-xylenes will be considered particularly. A close cut aromatic hydrocarbon fract on boiling in the range l38-140 0., obtained by fractional distillation of a solvent extract, will generally be found to be high in m-xylene and somewhat lower in p-xylene and very low in oxylene and ethyl benzene. Selective sulfonation of the fraction may be carried out at temperatures in the range 20 C. to 100 C. with about to strength sulfuric acid as sulfonating agent. The proportion of hydrocarbon feed and time of contactare also controlled to keep a substantial amount of the hydrocarbon feed unreacted. If desired, inert gases may be passed through the reaction zone 3 iorremoval ofwaterinmaintainingtheacidooneentration at a given level The unreacted and unabsorbed portion of the feed form a liquid rafllnate phase which is distilled or decanted from 4 The above data indicate that, as the acid sulfonating agent becomes diluted by absorbed and reacted xylenes, there is an increased concentration of unabsorbed p-xylene in the raiiinate. At

the extract solution phase that stratifies as a 5 the same time, however, the yield of ramnate didenser liquid layer when the reaction mixture is minishes to a low level faster than the increase of settled. p-xylene concentration in the rafiinate. Accord- The raiiinate removed from the add extract ingly, it is advantageous to increase the yield of solutionshows a higher freezing point than the rafllnate at the expense of purity in the initial initial hydrocarbon feed in proportion to the instages of selective sulfonation. creased concentration of p-xylene (P. P. 13 C.) W a it contains. By cooling the raillnate to a moderately low temperature, e. g., about 0' 0., p-xylene Increased proportions of the hydrocarbon ieed crystals are formed and are separated byflltratlon to the acid were used under comparative condifrom the mother liquor. l5 tions in obtaining the following test results:

As the p-xylene is removed from the raiilnate, T bk 2 the proportion of m-xylene is increased in the a mother liquor and the freezing point of the mother liquor becomes lower until it h imprac ghgi v01. Per a li" Freezing 1 5" tical to freeze out more p-xylene. At this point, csrbdn r ggg a. 3 p-xyleno in it is advantageous to subject the mother liquor Ad bsorbed Raillnate again to selective sulionation for extraction of a more m-xylene. Thus, the proeessis adapted for m 7 M 34 a continuous cyclic procedure in which the mother 2:1 so 17 '-1e 42 liquor is recycled for repeated sdeetive sulfona- 87 43 35 tion.

The acid extract phase removed from the raf- The above data show that increased p-xylene flnate phase on being subjected to steamdistillasegregation is obtained by increasing the ratio oi tion undergoes hydrolysis. The m-xylene sulhydrocarbon feed to the sulfonating acid. ionic acid is hydrolysed at a relatively lower tem- Further increased segregation may also be obperature than the p-xylenesuli'onk: acid. The tained by diluting the reaction mixture or the distillate recovered from the extract, steam disreaction products with low boiling parafiinic hytilled at 140' C.to 170 C.,isrlehinm-xylene; and drocarbons which are inert in the sulfonation, later when the temperature oftheresldualextract s.. y p s r h a s: nd in th course is raised to within the range 170-300 0., the of the sulfonation, the sulfonating acid may be distillate is richer in p-xylene. The p-xylenediluted by recycled sulionic acids. rich distillateissuitable fortmctiomlcrystallim- In r as d separation of p-xylene by havin tion at moderately low temperatmesaioneorwith increased conce t a o o pv e i the feed the railinate. sulfonated is evidenced in the following data:

Table 8 00 no lrreeiging Fr'eelzing Purity Per mod oi' l sd who oill fl i l iate filial? c'. 0. e m-xylenu. -ss 14 -4 as p, m-xy -1 es -o.s 70

Test data demonstrating the principles set Procedures in practicing the invention are forth are given in the following examples and p fied as fo lows: t A major volume proportion of an initial xylene m 1 55 cut containing 30% pand 67% m-xylenes is mixed and heated to C. with 95% HzSOs One volume of a Xylene i de ta 3 until at least one-fourth but not more than p-xylene and 67% m-xylene was mixed with one three-fourths of the xylenes ar absorbed. The volume of 95% H1804 at 30 C. to C. The unabsorbed xylenes, comprising preferably more mixture was agi a f r M 11 increased so than one-half the feed mixture, are separated as periods, and mples were taken of the ramnate a raflinate oil from the acid extract solution. and extract phas 11 137 18 y freezing poin The extract solution is subjected to steam dis- The following analytical results were obtained: tillation to obtain at least two main cuts, one

a Table 1 of these cuts being recovered at a stillpot temperature around 140' C. and below 170 C. and

v I P Ce v 1 Pace t d mow the other up to 200 C. An intermediate cut con- 1m taining, for example, 30 to 50% para xylene,

m ma may be sent back to the sulfonation zone. The

higher boiling main cut rich in p-xylene is adc. 'mixed with raillnate obtained from the selective 22 :2 *sulfonation for cooling to a moderately low temsLIIIIIII s; as --n so perature above -30 C. The precipitated p- 14 xylene crystals are removed by filtration and the mother liquor retaining above 30% p-xylene is recycled for selective sulionation.

In the separation of the isomers by" using only freezing and filtration on the mixture, an expensive and complicated method has to be used. The para xylene forms a eutectic with the meta xylene, thus making separation difiicult even at -53 C. or below.

With th present method both para and meta xylenes are recoverable completely in high purity without having to use low freezing temperatures at which the eutectic forms, thus making a considerable improvement over the prior art.

In the present process the isomer mixtures are treated cyclically in two main stages, selective sufonation-hydrolysis and fractional crystallization-filtration.

The feed for the sulfonation is, in general, a mixture of the isomers with a para xylene concentration of 30 to 50 volume per cent. Steps are taken to have most of the meta xylene sulfonated and extracted from the feed yet leave a substantial yield of raflinate enriched in para xylene. By controlled steam distillation hydrolysis of the acid extract separated from the raflinate, a relatively pure meta xylene fraction and a fraction suitably rich in para xylen for the fractional crystallifiation are recovered. Intermediate products obtained in the steam distillation may be recycled to the sulfonation stage.

For the fractional crystallization, the xylene mixture to be subjected to freezing should contain preferably 50 to 80 volume per cent of para xylene. In this stage, the feed, with or without a diluent, is cooled with agitation to crystallize out para xylene. The crystals are removed by filtration, washed, and fractionally melted to recover pure para xylene as a product. Some of the first meltings may be discarded or be added to feed subjected to freezing. By using only freezing points higher than the eutectic mixture freezing point, the mother liquor still contains a considerable proportion of para xylene. At freezing points of 12 to 30 C., the mother liquors free of solvent or diluent contain 30 to 50 per cent of para xylene and can be thus appropriatel used as feed to the sulfonation stage.

We claim:

1. The method of separating para xylene from meta xylene, which comprises predominantly sulfonating meta xylene in a feed mixture of said xylenes containing sufllcient para xylene to enable para xylene to be se arated therefrom by crystallization until at least one-fourth oi the xylenes remain unreacted in the mixture, separating a rafilnate fraction of the unreacted xylenes from an acid extract of the xylenes sulfonated in the mixture, heating the acid extract to a temperature in the range 140 C. to 170 C., steam distilling from the acid extract thus heated a meta xylene rich cut, heating residual acid extract of the steam distillation to a temperature in the range 170 C. to 200 C., steam distilling a para xylene rich cut from the thus-heated residual acid extract, freezing out para xylene crystals from the ramnate and said para xylene rich out until mother liquors of the crystallization retain at least 30% of para xylene, and using said mother liquors in the feed mixture for said sulfonating as hereinbefore stated.

2. The method of separating para xylene from meta xylene, which comprises predominantly sulfonating meta xylene of a xylene feed containing at least 30% para xylene until at least onefourth of the xylenes remain unreacted in the feed mixture, separating sulfonated xylenes of the feed from a raflinate having a higher para xylene concentration than the feed, steam distilling from the sulfonated xylenes a series of fractions having increasing para xylene content, recycling to the feed for sulfonation an intermediate fraction of medium para xylenecontent from said series, adding a higher boiling fraction rich in para xylene from said series to said raffinate, crystaliizing para xylene from the mixture of raftinate and the higher boiling steam distillation fraction, and recycling to the feed for sulfonation crystallization mother liquors in which at least 30% of xylenes present is para xylene.

3. A process for separating substantially pure mand p-xylenes from a mixture of isomeric xylenes including mand p-xylenes and containing sufficient para xylene to enable para xylene to be separated therefrom by crystallization which comprises sulfonating the mixture by heating it with at least equal volumes of sulfuric acid of '70-l00% initial concentration, maintaining the reaction mixture at a temperature of 20-100 C. during sulfonation for a period of time until at least one-four but not more than three-fourths of the xylenes are reacted and absorbed as an extract by the acid leaving a rafiinate containing p-xylene, separating the rafflnate and the extract, heating the extract with steam at a temperature between -170 C. to separate a m-xylene rich cut, heating the residual extract to -200 C. with steam to separate a p-xylene rich cut, combining the pxylene rich cut with the rafllnate to form a mixture containing relatively more p-xylene than was present in the initial xylene feed mixture, cooling the combined rafiinate and p-xylene cut to a temperature between -4 to -30 C. whereby a substantial crystallization oi the p-xylene results and recycling the mother liquor to the sulfonation step.

LELAND K. BEACH. CHARLES E. MORRELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,503,937 Cannon Aug. 5. 1924 1,940,065 Spannagel et a1. Dec. 19, 1933 2,393,888 Cole Jan. 29, 1946 2,398,526 Greenburg Apr. 16, 1946 OTHER REFERENCES 

1. THE METHOD OF SEPARATING PARA XYLENE FROM META XYLENE, WHICH OCMPRISES PREDOMINANTLY SULFONATING META XYLENE IN A FEED MIXTURE OF SAID XYLENES CONTAIING SUFFICIENT PARA XYLENE TO ENABLE PARA XYLENE TO BE SEPARATED THEREFROM BY CRYSTALLIZATION UNTIL AT LEAST ONE-FOURTH OF THE XYLENES REMAIN UNREACTED N THE MIXTURE, SEPARATING A RAFFINATE FRACTION OF THE UNREACTED XYLENES FROM AN ACID EXTRACT OF THE XYLENES SULFONATED IN THE MIXTURE, HEATING THE ACID EXTRACT TO A TEMPERATURE IN THE RANGE 140*C. TO 170*C., STEAM DISTILLING FROM THE ACID EXTRACT THUS HEATED A META XYLENE RICH CUT, HEATING RESIDUAL ACID EXTRACT OF THE STEAM DISTILLATION TO A TEMPERATURE IN THE RANGE 170*C. TO 200*C., STEAM DISTILLING A PARA XYLENE RICH CUT FROM THE THUS-HEATED RESIDUAL ACID EXTRACT, FREEZING OUT PARA XYLENE CRYSTALS FROM THE RAFFINATE AND SAID PARA XYLENE RICH CUT UNTIL MOTHER LIQUORS OF THE CRYSTALLIZATION RETAIN AT LEAST 30% OF PARA XYLENE, AND USING SAID MOTHER LIQUORS IN THE FEED MIXTURE OF SAID SULFONATING AS HEREINBEFORE STATED. 